HW Team2 - Fix the power in the elecCurrentPDE

HW2_Team2 elecCurrentPDE changes

• changed factor for postprocessing result of elecPower
• updated test Singlefield/ElecConduction/OhmsLaw:
  • added postprocessing result for elecFieldIntensity and elecPower
  • added output as h5,txt for elecPower
• updated h5ref file
• added readme with analytical calculation of elecpower for validation of the numerical result

Testsuite merge request: Testsuite!62 (merged)

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